Edit Mixamo animation in Blender
I realize this is an old question, but the best way I found for editing Mixamo's animations are these:


Create your own IK controls on top of it ( I usually import an already IK-rigged model into Mixamo, but you can apply IK to a fresh rig imported from Mixamo). For example, you can leave the rest of the animation as is, and just control the arm with your IK. If you've rigged the rest of the arms, legs, etc. then make sure to turn the IK influence down to 0 for these (otherwise the mixamo animation won't work).
Edit specific F-Curves. For example, to move the arm, you want to select either the upper arm or forearm bone, go to Curve editor, type "rotation" into the curve editor search box and try moving the whole curves up and down. You can also pick a point in time where you want to modify the movement, select a keyframe on the curve there, and move it up and down so that the hand's position is the way you want it. (enable proportional editing to also affect neighboring keyframes - otherwise your animation will only look right at that specific frame and snap right back at the other ones. You can also edit every single frame but I wouldn't do that, as it's too much work and you're bound to get jittery results)